1. Field of the Invention
The present invention relates to the field of integrated circuit digital-to-analog converters.
2. Prior Art
A simple digital-to-analog converter is comprised of a plurality of resistors coupled in series across a voltage reference, with a decoder receiving and decoding a digital input signal to control switches to couple the voltage of the selected node between resistors to the digital-to-analog converter output. A segmented resistor string digital-to-analog converter is similar in some respects, though contains first and second pluralities of resistors, each plurality of resistors being coupled in series to define two resistor strings. The first resistor string is coupled across a voltage reference. The second resistor string is coupled across two adjacent nodes in the first resistor string selected in response to the output of a decoder driven by the most significant bits of the digital input signal. The least significant bits of the input signal are decoded and used to couple the selected node between resistors in the second resistor string to the digital-to-analog converter output.
Many segmented resistor string architectures for digital-to-analog converters are well known in the prior art. These architectures include isolating the primary string from a secondary string by use of a pair of unity gain buffers, and applying a secondary resistor string directly across a primary string element and predicting the voltage shifts in the primary string voltages. Other similar, and more exotic, approaches are also known in the prior art.
Segmented resistor string digital-to-analog converters using a resistor string primary converter segment and a current source secondary. The primary converter segment provides conversion of the most significant bits to an analog voltage form and the current source provides conversion of the least significant bits to an analog current form. An output circuit combines the two into a single analog output. Various embodiments are disclosed, including an embodiment using a binary weighted current source and an embodiment using a thermometer code current source.